The invention relates to a method for thermally supported mechanical joining. The invention also relates to a device for carrying out the method.
For joining sheets, for example, resistance spot welding is known. In this method the sheets are melted locally and joined in the melted zone. Due to the structure changes occurring in the melting zone, the sheets withstand smaller mechanical loads in this zone.
For mechanical joining, methods are known like
clinch joining,
self-piercing riveting with semi-hollow rivets,
self-piercing riveting with solid rivets.
The mechanical joint is made at room temperature. A disadvantage of these methods is that the mechanical joining process leads to, in some cases, extreme, strain hardening in the joint zone. The appropriate devices as well as the tooling must be dimensioned to withstand high loads. Joining auxiliary parts such as semi-hollow rivets or solid rivets are deformed, even destroyed. Therefore, high-strength sheets can be employed to only a limited extent.
It is the objective of this invention to describe a method as well as a device that make possible the use of high-strength materials, also with an extremely high strain hardening degree as austenitic high-quality steels, for self-piercing riveting with semi-hollow rivet or solid rivet, or clinch joining, respectively.
According to the invention, the problem is solved by a method with the features mentioned in claim 1. Further, the problem is solved by a device with the features mentioned in claim 7. The subclaims present other useful developments of the invention.
The essential aspect of the invention is that a local and time limited heating of the parts to be jointed by electrical resistance heating takes place in the future deformation zone, whereby the heating is performed via the blank holder and die or elements arranged directly adjacent to said blank holder and die or directly preceding them, and the heating is initiated at the beginning or immediately before and is completed before the beginning of the forming in the deformation zone.
Advantageously, in a first version of the method, the heating takes place directly between a blank holder and a die, whereby the heating is initiated at the beginning of the working stroke For this, the blank holder is provided with an inclined bearing surface, and the die with a ring.
In an advantageous second version of the method, the heating occurs between an upper ring consisting of one or several contact shoes and a lower ring consisting of one or several contact shoes, which are arranged surrounding the blank holder and die each, whereby the heating is initiated at the beginning of the working stroke.
In an advantageous third version of the method, the heating occurs between a first electrode and a second electrode, which are assigned to a first station and the forming is carried out in a subsequent second station, whereby the heating is initiated immediately before the beginning of the forming.
The heating does not primarily affect the tooling or the joining auxiliary parts, but the heating covers that zone in the parts to be joined in tat the highest degree of deformation occurs. Heating is completed before the forming process begins, and starts either after the beginning of the working stroke or immediately before the working stroke of a station preceding the forming. Hence, heating can be easily controlled and the energy input limited to a minimum. After forming cooling of the joint immediately starts. This is achieved by inputting a minimum amount of heat and exactly timing the blank holder stroke/heating/forming.
For the realization of the method the blank holder and the die or elements arranged directly adjacent to said blank holder and die or directly preceding them are designed as poles for electrical resistance heating, whereby the blank holder, die and contact shoe are provided with appropriate bearing surfaces so that current lines extend through the parts to be joined in a locally restricted extent which limit the range of heating.
In a first device the blank holder is provided with an inclined surface and the die with a ring-shaped elevation.
In a second device an upper ring of one or several contact shoes that is arranged surrounding a blank holder, and a lower ring of one or several contact shoes that is arranged surrounding a die, are provided, whereby the bearing surfaces of the contact shoes are located very close to the blank holder and the die thus limiting the range of heating.
In a third device electrodes for heating of the parts to be joined are provided in a station that is arranged preceding the forming station.
The advantage of the invention is that the known self-piercing riveting with semi-hollow rivets or solid rivets, or the clinch joining can also be employed with high-strength materials, also with an extreme degree of strain hardening, whereby no negative effect on the quality of the joint due to overstressing of the joining auxiliary parts or basic material, respectively, occurs during forming. A radial prestress at the joining point due to cooling at the joint increases the strength of the joint.